1. Field of the Invention
The present invention relates to the operation of steam turbines and electric power plants and more particularly to the implementation of adaptive control techniques to assure the positive and accurate control of steam turbines and electric power plants.
2. Description of the Prior Art
In order to meet the increasing demands for the generation of electrical power, electric plants including boilers and turbines of increased size have been incorporated into power generating systems including an increasing number of interconnected plants. As larger units and greater numbers of such units are placed into service to meet ever-increasing power energy requirements, the control of power generation of each unit required improvement in order to achieve good frequency control over the entire system. In addition to systems requirements, there was a strong requirement that new methods be developed to extract energy from the boiler as well as to set limits by which the boiler could be operated safely and efficiently. As discussed in the article, "System Design Considerations For Advanced Utility Unit Control," by T. A. Rumsey and D. L. Armstrong, presented at the 14th Annual Southeastern ISA Conference, April of 1968, the required improved control of power generation is efficiently accomplished by achieving a close coordination of the boiler and turbine controls. As suggested in this article, the controls for the boiler and turbine are placed in parallel in a manner similar to the boiler follow system, except that the steam pressure is varied to take advantage of the energy stored in the boiler. The turbine regulates steam pressure, but with a changing set point derived from the error between load demand and actual unit output. If the load demand is higher than the actual unit output, the signal applied to the pressure controller calls for a lower steam pressure, thus opening the governor valve and temporarily increasing megawatts as the pressure drops. The same signal applied to the pressure controller effecting a lower pressure in response to detection of a megawatts output below the required demand level, increases the boiler inputs (water, air and fuel). This control action continues until the megawatt error is zero, at which time the steam pressure is at its normal value. Such integrated control techniques have been applied to once-through, supercritical boilers and to drum-type sub-critical boilers.
A significant aspect of the integrated control of turbines and boilers is the use of feed forward control techniques to minimize interaction and to extract the best possible dynamic response. Generally, such feed forward control is effected by applying load demand signals from either the ADS, a computer, or a manual operator control, simultaneously to the boiler and turbine. The advantages of such a control means that subloop process changes are made simultaneously with load changes before subloop errors exist. Feedback controllers are used as a final trim on the process subloop to correct for minor non-linearities and static effects. The trimmed or modified load references are applied, in turn, to the boiler and turbine controls. As a result, it is possible to extract energy more efficiently from the boiler of an individual unit, whereas on a system level, each of a plurality of units may be operated so as to maintain system frequency integrity.
As described in an article entitled "Digital Control Techniques For Plant Applications" by Theodore Giras and Robert Uram, Combustion, March 1969, such coordinated schemes of generating power require improved techniques of digital control including nonlinear feed forward characterization of major plant variables such as load demand, boiler demand, feedwater demand, fuel demand and air demand; calibration of the feed forward control action by measured variables such as pressure, temperatures and flows; adaptive controllers sensitive to real plant variables and adjusted to operate over the entire range from no load to full load; minor-loop feed-back control which is coordinated throughout the entire system; and finally, logical interaction of all control loops to ensure bumpless transfer from manual to automatic, and from automatic to manual, modes of operation.
The wide range of controllability required for the steam plants of today suggests the use of high-speed digital controllers to implement the sophisticated control philosophy necessary for proper operation.
There are a number of basic requirements which a digital system must satisfy in order to control a complex process. First is the ability to alter or modify the control package easily and quickly in the field to accommodate process dynamic characteristics which could not be anticipated early in the design. In addition to this block flexibility, the digital package must be designed so that process parameters can be changed quickly and accurately. Thus, plant gains, biases, set points, limits, time constants and other important system data must be arranged in the computer storage in such a fashion that inexperienced field personnel may adjust these values literally at will. This is of paramount importance, for as more is learned over a period of time in controlling a plant with a computer, refinements in the control system must be made to improve operating efficiency and reliability.
Another major requirement of a digital system is careful selection of the computing schemes used in the various controllers and functional blocks. Since all implementation within the computer must ultimately be done with numerical methods, the general formulation and selection of any algorithm structure becomes quite critical. Thus, the numerical schemes for integration, differentiation, smoothing, and characterization must be carefully selected to assure proper control action, and yet be simply and easily programmed.
Dynamic or integrated controllers have been used to implement the various methods of calculation to achieve the desired control action. Such controllers may take the form of a reset, rate, proportional plus reset, proportional plus rate, and proportional plus reset plus rate-type controller. Such controllers may be used either on-line or off-line to provide a direct output for control or to provide a trim of a reference demand. As described in the article entitled "Hybrid Digital-Analog Power Plant Control" by Guy E. Davis, Jr., ISA Transactions, 1970, such controllers may be used in conjunction with a manual/auto station for the control of a typical valve within a boiler. While operating on Manual, the output of the transducer associated with the valve is applied to a computer, which must track the operator's adjustment to the control. The term "track" connotes the process by which the computer forces its calculated output to match the present manual station demand for valve position and transfer from Manual to Auto without bumping the process. In order to ensure a bumpless transfer between the Manual and Auto Modes of operation, the operator may balance the process to the correct setting before transferring from Manual to Auto. For that type of control, the operator uses a null meter on the manual station to determine process balance. More recent electronic systems use tracking amplifiers to modify the demand signal to agree with the actual operating set point. After balance is achieved and transfer occurs, the tracking amplifiers' off-set is made to decay to a neutral value. This method was applied in a computer control program as one form of bumpless transfer. The use of controllers in such bumpless transfer systems has proved effective in boiler control systems.
Considering that one of the purposes of operating in a coordinated mode is to achieve a more accurate frequency control over the power plant so that the single power plant may be coordinated more effectively with the plants of the entire system, it is desirable to effect a more positive control over the various parameters of turbine and boiler operation whereby the power generated and its frequency likewise are positively controlled. To accomplish this overall objective of improved power generation, it is desirable to provide new and improved controllers of increased flexibility in terms that their time constants or gains may be adjusted readily and that their response to inputs may be controlled readily as to rate of change and as to accuracy of response according to a desired function to varying inputs.